The invention relates to monitoring airplane engines and more particularly gas turbine airplane engines. A preferred but non-limiting particular application of the invention lies in monitoring two-spool turbojets.
Monitoring the operation of an engine serves advantageously to anticipate engine stoppages associated with component failure. It also serves to prepare maintenance operations on the engine, given knowledge in advance of the problems that need to be solved.
In conventional manner, such monitoring relies on processing signals recorded by sensors sensing the operation of the engine, e.g. sensors such as vibration, speed, temperature, or pressure sensors. As a function of the characteristics of the engine and of the monitored members, it is possible to identify the beginning of a component failure and to program maintenance action on that component before its failure is complete and leads to the engine stopping.
In particular with two-spool turbojets, it can also be useful to know the angular positions of the various rotors of the turbojet (e.g. rotary shafts), in order to perform more accurate analysis of identified failures (e.g. changes in a blade of a rotor presenting unbalance).
For this purpose, it is known to use a speed sensor fitted with a toothed wheel secured to the rotor and provided with a tooth that is different from the others. By way of example, such a wheel is described in document EP 1 777 526. The different tooth of the toothed wheel generates a signal that is different from the signal generated by the other teeth, thus making it possible to identify the angular position of the rotor at instants when said signal is detected by the speed sensor.
Nevertheless, in order to determine the angular position of a rotor in this way by analyzing signals measured by such a speed sensor, it is necessary for signal measurement to be performed directly on the rotor for which it is desired to know the angular position.
Although it is easy to position the speed sensor fitted with a toothed wheel on a shaft of a low pressure rotor in a two-spool turbojet, the same is not true for the shaft of the high pressure rotor of said turbojet since it is difficult to access that shaft. Installing a speed sensor fitted with a toothed wheel on a shaft of a high pressure rotor would then require having recourse to devices that are complex and expensive, such as for example the device described in document U.S. Pat. No. 4,075,562, where such a device is difficult to incorporate in the context of a set of multiple rotors.